1. Field
The present invention relates generally to data communication, and more specifically to techniques for selecting rate for a wireless (e.g., OFDM) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may implement orthogonal frequency division multiplex (OFDM) modulation, which may be capable of providing high performance for some channel environments. In an OFDM system, the system bandwidth is effectively partitioned into a number of (NF) frequency subchannels (which may be referred to as sub-bands or frequency bins). Each frequency subchannel is associated with a respective subcarrier (or frequency tone) upon which data may be modulated. Typically, the data to be transmitted (i.e., the information bits) is encoded with a particular coding scheme to generate coded bits, and the coded bits may further be grouped into multi-bit symbols that are then mapped to modulation symbols based on a particular modulation scheme (e.g., M-PSK or M-QAM). At each time interval that may be dependent on the bandwidth of each frequency subchannel, a modulation symbol may be transmitted on each of the NF frequency subchannels.
The frequency subchannels of an OFDM system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs). Each transmitted modulation symbol is affected by the frequency response of the communication channel at the particular frequency subchannel via which the symbol was transmitted. Depending on the multipath profile of the communication channel, the frequency response may vary widely throughout the system bandwidth. Thus, the modulation symbols that collectively form a particular data packet may be individually received with a wide range of SNRs via the NF frequency subchannels, and the SNR would then vary correspondingly across the entire packet.
For a multipath channel having a frequency response that is not flat or constant, the number of information bits per modulation symbol (i.e., the data rate or information rate) that may be reliably transmitted on each frequency subchannel may be different from subchannel to subchannel. Moreover, the channel conditions typically vary over time. As a result, the supported data rates for the frequency subchannels also vary over time.
Since the channel conditions experienced by a given receiver are typically not known a priori, it is impractical to transmit data at the same transmit power and/or data rate to all receivers. Fixing these transmission parameters would likely result in a waste of transmit power, the use of sub-optimal data rates for some receivers, and unreliable communication for some other receivers, all of which leads to an undesirable decrease in system capacity. The different transmission capabilities of the communication channels for different receivers plus the time-variant and multipath nature of these channels make it challenging to effectively code and modulate data for transmission in an OFDM system.
There is therefore a need in the art for techniques to select the proper rate for data transmission in a wireless (e.g., OFDM) communication system having the channel characteristics described above.